50 Gal water heater, 10-15 min shower?

jfrotten probably is right turn it off and check the elements. Even if they check good they are most likely covered with a crust of sediment. If you replace them call around and get "lifelong heating elements" (made by State Industries in TN, I bought mine at True Value Hardware). I replaced them about every two years (hard water) from the time it was new. After changing to these I haven't changed one in eight years. I keep them both set at 129.